Idea/Prediction: A new villain crashes the Equestria Games a la Bane or Amon and is defeated by whatever is in the mystery box (that will be unlocked by what I call "The MacGuffins of Harmony")

Potential support for this theory is the fact that the S4 finale is a two-parter named "Twilight's Kingdom" with the description for the first part being "Princess Celestia chooses Discord over Princess Twilight to help track down a villain" and before that episode is the "Equestria Games" episode which could (and should) end on the cliffhanger of a villain crashing the games.